The Navstar Global Positioning System (GPS) is used to determine exact geographic position (i.e., latitude, longitude and altitude) absolute velocity as well as the exact time. The navigation device must calculate the position, velocity and the time by determining distance and relative velocity to a series of satellites. The velocity of the receiver is calculated from the doppler frequency shift of signals transmitted from the satellites. This distance is called range and the doppler shift yields range rate.
A GPS receiver must receive signals generated from a satellite about 11,000 miles away. A GPS satellite transmits about a 6 watt spread spectrum signal. The satellite and receiver, therefore, has to employ spread spectrum techniques to differentiate the signal from the noise. "Spread spectrum" means that the frequency or instantaneous phase of the signal being transmitted changes as a function of time. Using correlation techniques, the receiver can track the spread spectrum signal coming from the satellite by estimating a duplicate image of the signal. A precise match of the satellite's spread spectrum produces a potential signal processing gain of up to 53 decibels. The use of spread spectrum techniques is essential to receive 6 watts of energy transmitted 11,000 miles away. At the antenna, the GPS signal is about 20 dB below ambient cosmic noise.
Prior GPS digital receivers performed signal processing functions in the analog domain thereby requiring many expensive discrete components, some of which needed to be tuned to match the circuitry. A common cost/performance tradeoff was to multiplex or sequence one or two channels.
By digitizing the GPS signal at the final IF stage and performing all channel signal processing functions digitally in a low cost CMOS IC, GPS receiver performance is greatly improved and cost is reduced. Such a system is disclosed in concurrently filed copending patent application Ser. No. 07/662,585, filed Feb. 28, 1991, now U.S. Pat. No. 5,347,284, entitled "System and Method for a Digital Navigation Satellite Receiver," which patent is hereby incorporated by reference herein.
While the IC presents significant benefit to a GPS receiver, it also presents several problems in building an IC that is low cost and consumes little power. Digitizing the GPS signal at the final IF requires high frequency operation for a signal processing device, i.e. 40 to 60 megahertz for a double sideband GPS signal. Features in a GPS signal processing device which improve performance in hostile environments include full null zone signal integration, multiple correlators, anti-spoofing, and a search processor for fast acquisition.
When all these features are combined in one chip, it is very difficult to meet power consumption goals for the device and to minimize the gate count of the device, thereby minimizing its cost. In addition to the cost of the IC, there is a finite limit to the number of gates which can be put on the IC and thus designing a circuit that will fit on a single IC is difficult.